1. Technical Field
The present invention relates to an inkjet printer and a printing method using the inkjet printer.
2. Background Art
Conventionally, there is known an inkjet printer in which ink is ejected from a print head onto a print medium put on a platen while reciprocating the print head in a right-left direction so as to print the print medium. As one of such inkjet printers, there is a printer of a type ejecting ultraviolet curable ink (hereinafter, referred to as UV ink) having a property that it is cured when irradiated with ultraviolet light. Since the UV ink has excellent weather resistance and excellent water resistance, the UV ink allows printed matters to be used as outdoor advertising posters or the like. Therefore, the UV ink has the advantage that the use of printed matters printed with UV ink dramatically increases the range of purposes as compared to printed matters printed with water-soluble ink. Generally, such an inkjet printer of a type ejecting UV ink is provided with an ultraviolet light irradiation device for curing the UV ink deposited on a print medium. In recent years, an inkjet printer has been developed in which an ultraviolet light emitting diode (hereinafter, referred to as UVLED) is used as a light source for emitting ultraviolet light in the ultraviolet light irradiation device.
As an example of the conventional print unit, a print unit 500 is shown in FIG. 10A. For convenience of explanation, directions indicated by arrows shown in FIG. 10A will be defined as forward, backward, leftward, and rightward directions, respectively in the following description. The print unit 500 includes mainly a print head 510 which ejects UV ink, a right ultraviolet light irradiation device 520R, a left ultraviolet light irradiation device 520L, and a carriage (not shown) on which these are disposed. Inside the right ultraviolet light irradiation device 520R and the left ultraviolet light irradiation device 520L, UVLEDs are arranged so as to radiate ultraviolet light downwardly and are disposed and fixed on the right and left sides of the print head 510. The right ultraviolet light irradiation device 520R and the left ultraviolet light irradiation device 520L are designed to have a width in the front-back direction which is substantially the same as the width in the front-back direction of the print head 510. The print head 510 includes, for example, print heads for respective colors (not shown) such as a magenta print head, an yellow print head, a cyan print head, and a black print head.
To conduct printing on a printing line 508 of the print medium 501 by using the aforementioned print unit 500, UV ink droplets are ejected from the respective nozzles of the print heads for respective colors so that the UV ink droplets are superposed in predetermined patterns on a printing line 508 while reciprocating the print unit 500 above the printing line 508 a predetermined number of passes. During this, the right ultraviolet light irradiation device 520R and the left ultraviolet light irradiation device 520L emit ultraviolet light. The printing line 508 is irradiated with the ultraviolet light so as to cure the UV ink deposited on the printing line 508.
FIGS. 10B, 10C are sectional views showing states that LTV ink droplets ejected from the print head 510 are deposited on the printing line 508 as mentioned above. FIG. 10B shows a state that uncured UV ink droplets 512 are ejected and deposited at the current pass on completely cured UV ink droplets 511, which were ejected and deposited at the last pass on the printing line 508 and which were irradiated with ultraviolet light and thus completely cured. Since the UV ink droplets 511 are completely cured, the affinity of the uncured UV ink droplets 512 for the completely cured UV ink 511 are poor so that the uncured UV ink droplets 512 are deposited in a raised shape like beading because of surface tension. After the uncured UV ink droplets 512 are deposited in a beading state, the uncured UV ink droplets 512 spread very little before irradiation with ultraviolet light because of poor affinity and is then completely cured in this state by irradiation with ultraviolet light.
On the other hand, FIG. 10C shows a state that uncured UV ink droplets 514 are ejected and deposited at the current pass on uncured UV ink droplets 513, which were ejected and deposited at the last pass on the printing line 508 and which were not cured (or cured very little). The affinity of the later uncured UV ink droplets 514 for the prior uncured UV ink droplets 513 are good so that, after the later uncured UV ink droplets 514 are deposited in a beading state, the later uncured UV ink droplets 514 are mixed with the prior uncured UV ink droplets 513 and thus bleed. The later uncured UV ink droplets 514 and the prior uncured UV ink droplets 513 are mixed so as to form a mixed UV ink 515. The mixed UV ink 515 is irradiated with ultraviolet light and is thus completely cured. To prevent UV ink from bleeding as mentioned above, for example, JP-A-2004-276584 discloses an arrangement in which, after the surfaces of ink droplets deposited on a recording medium 2 are cured by ultraviolet light emitted from first light irradiation devices 17, 18, 19, and 20, the ink droplets are completely cured by ultraviolet light emitted from a second light irradiation device 21.
By the way, for printing on the print medium 501 by the print unit 500, it is preferable that UV ink droplets deposited and superposed on the print medium 501 are not mixed and thus do not bleed, but the UV ink droplets spread and are thus leveled. In this case, the print medium 501 with desired printing (desired printed matter) can be obtained. However, when the uncured UV ink droplets 512 are superposed on and adhere to the completely cured UV ink droplets 511 as shown in FIG. 10B, the completely cured UV ink droplets 511 and the uncured UV ink droplets 512 are not mixed and thus do not bleed, but the completely cured UV ink droplets 511 reject the uncured UV ink droplets 512 so that the uncured UV ink droplets 512 may be cured by irradiation with ultraviolet light in the state remaining a raised shape like beading on the surface of the completely cured UV ink droplets 511. As compared to the desired printed matter, the printed matter in which UV ink droplets are cured in the state remaining the beading shape may have poorer print quality because reflection of light from the printed matter may differ so as to cause difference in vision.
When the later uncured UV ink droplets 514 are superposed on and adhere to the prior uncured UV ink droplets 513 as shown in FIG. 10C, the later uncured UV ink droplets 514 may be mixed with the prior uncured UV ink droplets 513 and thus bleed so that the UV ink droplets may be cured by irradiation with ultraviolet light in the mixed and bleeding state. As compared to the desired printed matter, the printed matter in which UV ink droplets are cured in the mixed and bleeding state may have poorer print quality because a mixed and bleeding portion of the printed matter has different color in vision.